1. Field of the Invention
The present invention relates to a rapid cutoff device and operation method for SCR AC switches. More particularly, the present invention relates to a bi-directional rapid cutoff device and operation method for SCR AC switches.
2. Description of the Related Art
Generally, switch elements mainly include active switch elements and mechanical switch elements. However, when the mechanical switch element is applied in an AC circuit, there is a need of providing adequate capabilities of switching off and eliminating electric arc.
In addition, in switching-off operation, the mechanical switch element will generate electric arc which will damage the contact points and reduce the service life. In order to avoid the aforesaid drawbacks, the active switch elements have replaced the mechanical switch elements. MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) of the active switch elements are generally applied to provide a function of hard switching. In switching-off operation, the active switch element, however, applied in both of the AC circuit and the DC circuit can effectively and rapidly cut off the current therein and further has an advantage of resulting in no electric arc. Even though the active switch element has the advantage of rapidly switching, it also has characteristics of low ratings of voltages and currents and cannot endure large inrush currents. Accordingly, the active switch element has a drawback of low reliability.
Furthermore, thyristor switch elements are excellent high-power elements which have high voltage rating and high inrush current capability. The thyristor switch elements further have several advantages of a wider range of applications, longer service life, higher reliability, rapid switching speed, minimized dimensions, easy and economic maintenance, lower power consumption and low manufacture cost. However, when the thyristor switch element is applied in the AC circuit, the thyristor switch element is completely cut off by decreasing its current to a near-zero value after stopping a drive signal. Namely, the thyristor switch element cannot be cut off immediately by stopping the drive signal.
By way of example, U.S. Pat. Publication No. 20120086427, entitled “Alternating Current/Direct Current Two-way Switch,” corresponding to Taiwanese Pat. Publication No. 201106623, discloses an AC/DC two-way switch. The AC/DC two-way switch connects between an AC power source or a DC power source and a load to control a power supply to the load. The AC/DC two-way switch includes: a bi-directional semiconductor switch element having a transistor structure connected in series between a power source and the load; a rectifying unit connected in parallel between the first input terminal and a second input terminal; a power source unit for converting an output voltage from the rectifying unit into a stable voltage; and a controller for controlling an entire operation of the AC/DC two-way switch. The AC/DC two-way switch further includes a driving unit for turning on the bi-directional semiconductor switch element in response to a control signal.
However, the above AC/DC two-way switch applied in a DC power distribution system can avoid occurrence of electric arc in switching on and off operation. The above-mentioned publication is incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the situation of the art.
Another U.S. Pat. No. 7,079,363, entitled “Hybrid DC Electromagnetic contactor,” discloses a hybrid DC electromagnetic contactor. The hybrid DC electromagnetic contactor includes: a power unit for supplying a certain power voltage; a main contact point of a breaking switch for providing a supply path of the power voltage by being switched in accordance with a voltage apply to an operational coil; a switch for providing a supply path of the power voltage according to a gate signal; a snubber circuit for charging voltage at the both ends of the switch in turning off of the switch and being applied-discharged an electric current when the charged voltage is not less than a certain voltage; and a discharge current removing unit for removing the discharge current by providing a discharge current path to a load block in turning off of the switch, it is possible to minimize a size of leakage current when the main contact point and the semiconductor switch are turned off.
However, the hybrid DC electromagnetic contactor provides the semiconductor switch parallel connected with the mechanical contact switch such that it can avoid occurrence of electric arc in switching on and off operation. The above-mentioned patent is incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the situation of the art.
As is described in greater detail below, the present invention provides a rapid cutoff device and operation method for SCR AC switches. A thyristor AC switch electrically connects between a first AC circuit and a second AC circuit. When the thyristor AC switch cuts off, a first switch is operated, according to a first direction of an AC current, to conduct a first capacitor which is charged by the AC current, or alternatively, a second switch is operated, according to a second direction of the AC current, to conduct a second capacitor which is charged by the AC current. Forcibly, a current of the thyristor AC switch is rapidly lowered for approaching a zero value, thereby rapidly cutting off the thyristor AC switch. Advantageously, the present invention can accomplish rapidly cutting off the thyristor AC switch and is unlike an operation method of the conventional thyristor AC switch which requires the current of the thyristor AC switch gradually decreasing to approach a zero value and then cutting off the thyristor AC switch.